In a laminated battery in which a plurality of power generating elements is laminated, it is preferable to detect an internal resistance as accurately as possible. For example, in a fuel cell, if the internal resistance is detected, then a degree of wetness of an electrolyte membrane can be known. In a case where the internal resistance is high, the degree of wetness of the electrolyte membrane is low and rather dry. In a case where the internal resistance is low, the degree of wetness of the electrolyte membrane is high. In a fuel cell, operating efficiency changes depending on the degree of wetness of the electrolyte membrane. Thus, by controlling the operation in accordance with the degree of wetness of the electrolyte membrane estimated on the basis of the internal resistance, a wetness condition of the electrolyte membrane can be constantly maintained in an optimal state.
In order to detect the wetness condition of the electrolyte membrane, a technology is known in which an alternating current is supplied by an external circuit to a fuel cell that is connected to an external load to measure an impedance of the fuel cell. However, with respect to the external circuit that supplies the alternating current, the battery and the load that is connected to the battery form a parallel circuit. Thus, in a case where the supplied current flows to the load other than the battery, there is a possibility that the internal resistance of the battery cannot be measured accurately. An apparatus for solving this problem is disclosed in WO2012/077450A.